1 . Technical Field
The present invention is directed to a brake arm and pad mount for a bicycle brake, and more particularly toward an adjustable brake arm and brake shoe mount for a cantilever brake.
2. Background Art
There are two common types of brake shoes for bicycles. One is known as a cantilever brake and the other is known as a caliper brake.
A cantilever brake generally includes a pair of arms mounted pivotally at their lower ends to a pair of bosses that are attached to a bicycle fork or frame on both sides of a wheel at a height below the wheel rim. The brake arms are connected by a straddle cable at their top ends. A brake pad or shoe is mounted to each arm at the height of and in facing relation to the rim. During actuation of the brake, the straddle cable is pulled by a brake cable attached to a remote lever on the handle bar which causes the brake arms to swing toward each other and thus the brake pad to contact the rim.
A caliper brake generally includes a pair of arms pivotally mounted at their intermediate portions to a pair of bosses attached to a bicycle fork or frame on both sides of a wheel at a height above the wheel rim. A brake pad is mounted to each arm at its lower end in facing relation to and at a height of the rim. On a center-pull type cantilever brake, the top portions of the arms criss-cross each other and are connected by a straddle cable at their top ends. A brake cable is connected at one end to the straddle cable and at the other end to a remote lever mounted on a handle bar of the bicycle. Actuation of the brake lever pulls the mid-point of the straddle cable and causes the upper ends of the brake arms to swing toward each other causing the brake pads to make contact with the rim. Caliper brakes also include cam-operated caliper brakes and parallelogram-linkage-operated type caliper brakes which will not be discussed in detail. The key point to understand is that cantilever brakes and all types of the caliper brakes function by placing a brake pad into contact with the rim of a bicycle.
When brake pads are not aligned to engage a wheel rim with the pad braking surface flush, proper braking performance is inhibited and the rider of the bicycle is at serious risk. Proper brake pad alignment can be achieved by a non-adjustable mounting mechanism if the brake assembly is to be used on a particular bicycle with a particular wheel rim. However, since different makes and models of bicycles differ in their brake boss positioning and wheel rims vary in their widths and the angle of the side walls of the rims, an adjustable mounting mechanism is necessary to allow brakes to be used on a variety of bicycle models using different rims. Further, even if a brake assembly is to be used only on a specific bicycle model with a specific rim, the brake pads still need to be adjusted to compensate for wear resulting from repeated normal use.
In view of the many advantages of an adjustable brake pad mount, they are commonly used with bicycle brakes. A typical mounting mechanism known in the art comprises a bolt reception bore penetrating through a shoe support portion of a brake arm length-wise of the bicycle and a support bolt loosely inserted through the bolt reception bore. The support bolt has a threaded end projecting from one end of the bolt reception bore for engagement with a nut, and an enlarged end or head projecting from the other end of the bore. The bolt head is formed with a cross bore extending transversely of the bolt. The brake pad assembly has a brake pad post inserted into the bolt cross bore.
With the pad mounting structure described above, when the nut is tightened, the brake pad post of the brake pad assembly is pressed transversely against the bolt cross bore, thereby fixing the shoe relative to the brake arm. When the nut is loosened, the support bolt and the brake pad post become loose for adjusting movement or rotation. For example, the brake pad post may be slidingly moved relative to the bolt cross bore for horizontal adjustment toward or away from the wheel rim, the brake pad post being further rotatable within the bolt cross bore about its axis for orientation adjustment. Moreover, the support bolt may be rotated within the bolt reception bore for a second orientation adjustment. If necessary, the bolt reception bore may be elongated vertically and cross-sectioned to provided vertical adjustment of the shoe position. This provides for adjustment of the degrees of pad movement, known as the "degrees of freedom".
For the sake of clarity, FIG. 1 illustrates the five degrees of pad adjustment freedoms will be defined as follows:
A. The "height"--the height of the pad relative to the rim; PA1 B. The "toe"--the angle the pad abuts the rim relative to its length; PA1 C. The "extension"--the axial position of the brake pad post relative to the brake arm; PA1 D. The "twist"--the rotation of the brake pad post about its own axis; and PA1 E. The "pad attitude"--the angle the pad abuts the rim relative to its width.
In the manner described above, the prior art brake pad mounting mechanism allows the brake pad to have five degrees of freedom and thus proper brake pad alignment to any rim on any bicycle. However, since the brake pad does not always require alignment in every degree of freedom, the multiple degrees of freedom are adjustable by loosening of a single bolt often causing extreme difficulty in actually aligning a brake pad. For example, if only the height needs to be adjusted by moving the support bolt within the reception bore, upon loosening the mounting nut, both the shoe post and the support bolt may come loose and may move unintentionally, thus nullifying all previous adjustments. In fact, it is usually necessary to readjust a brake pad in every adjusting freedom if any one of the freedoms is in need of alignment. Total realignment of all freedoms is inevitably required if the brake pad assembly is to be replaced.
Furthermore, when the mounting nut is tightened to secure the brake pad, the support bolt can tend to rotate unpredictably with the torque applied to the nut and thus can throw the brake pad out of alignment which has just been painstakingly achieved. Consequently, the brake pad is to be repeatedly aligned until by chance the support bolt happens not to rotate with the torque applied to the nut. Moreover, since the brake post and the support bolt become loose in their bores at the instant the mounting nut is loosened, the prior art mounting mechanism does not allow for incrementally adjusting the brake pad for fine tuning the alignment. Consequently, it is quite common for cyclists to lose patients in adjusting brake pads, thus increasing the risk that users will leave the brake pads improperly adjusted while riding the bicycle, increasing the risk of injury.
In view of these adjustment problems, at least three improvements to the prior art brake pad mechanisms have been proposed. One of the improvements is disclosed in Japanese Utility Model Publication No. 55-47740 and comprises a pressure plate that mounts on the support bolt between the pad post and the pad mounting portion and presses directly on the pad post via the force of a compression of a coil spring accommodated in a recess of the pad mounting portion of the brake arm.
Upon loosening the mounting nut of the mechanism for adjustment, since the compression spring constantly urges the pressure plate into pressing contact with the pad post and thus holding it against the cross bore of the support bolt, the movement of the pad post relative to the cross bore is frictionally restrained. On the other hand, the pad post may be forcibly adjusted if sufficient manual force is applied to overcome the friction imparted to it via the compression spring. Thus, this mechanism allows the height, toe and attitude to be adjusted without affecting the extension and twist, thereby simplifying the adjusting operation while retaining the benefits of the diverse freedoms.
However, the pressure plate, being merely pressed against the pad post via the compression spring, may be moved away from the pad post if an operator's finger accidentally presses down on the pressure plate while making adjustments. The pad post is then put back into its loose state relative to the cross bore and results in the same problem as described above in connection to the typical prior art.
Another improvement is proposed in U.S. Pat. No. 4,938,318 to Ishibashi. This improvement includes a pad post retainer that slips over the cross bore of the support bolt to be provided with a post retaining hole for alignment with the cross bore for press-fitting the pad post on a brake pad assembly as inserted into the cross bore. Upon loosening the mounting nut of the above mechanism for adjustments, since the pad post retainer is press-fit into the pad post and is inseparable from it and the support bolt, the movement of the pad post relative to the cross bore is frictionally restrained by the post retaining hole. On the other hand, the post may be forcibly adjusted if sufficient manual force is applied to overcome the friction between it and the post retaining hole. Since this pad mounting mechanism allows the height, toe and attitude to be adjusted without affecting the extension and twist, it simplifies the adjusting operation in the same manner discussed above with respect to Japanese Utility Model Publication No. 55-47740.
However, the press-fitting of the post retaining hole to the pad post inevitably will deteriorate over repeated adjustments of the brake pad, thereby, the pad post is put into its loose state relative to the cross bore, resulting in the same problems described above in connection with the prior art.
Both of these improvements to prior art shoe mounting mechanisms discussed above offer no advantage when compensating for brake pad wear or replacing the brake pad, in which case it is the height, toe and attitude are proper and only the extension and twist need to be adjusted. For example, upon loosening the mounting nut for mounting the twist or extension, the support bolt immediately becomes loose in the reception bore and may move unintentionally and thus annul proper height, toe and attitude adjustments. Further, these improvements do not address the problems of torque induced annulment of the just made alignment of the brake pad when tightening the main nut, or the lack of incremental adjustability of the brake pad for fine-tuning its alignment to perfectly match the rim's braking surface.
An improvement over both of these structures in incorporated in the Tri-Align.TM. brakes sold by Avid Enterprises, Inc., assignee of the present invention. The Tri-Align.TM. brake isolates the extension and twist adjustments so that a brake pad can be readily removed without having to reset the height, toe and attitude. However, if a user is to adjust either of the height or toe, the attitude is subject to inadvertent adjustment. Because attitude is a critical adjustment and difficult to achieve, this is a disadvantage of the Tri-Align.TM. brake pad adjustment structure notwithstanding the Tri-Align.TM. brake pad adjustment structure's many improvements over the prior art.
One additional problem with prior art pad adjustment mechanisms is that typically the point of attachment of the brake arm to the brake attachment boss (known as the "fulcrum" herein) is fixed and the height of the pad is varied by raising and lowering a brake pad adjustment structure relative to the fulcrum. Each brake has an optimal distance between the brake pad and fulcrum. If the attachment boss is too high, the brake pad adjustment must be moved closer to the fulcrum, thereby increasing the amount of leverage, and giving the brake a "soft" feeling. Conversely, if the adjustment boss is too low relative to the rim, the brake pad attachment assembly must be moved up, increasing the length between the fulcrum and the point of contact, thereby decreasing the amount of leverage and therefore requiring more force to actuate the brake.
The present invention is directed toward overcoming one or more of the problems discussed above.